Wireless communication systems include long term evolution (LTE), LTE-A, and LTE-A-beyond systems. A modern wireless communications system may include a plurality of NodeBs (NBs), which may also be referred to as base stations, network nodes, communications controllers, or eNBs (enhanced NBs), and so on, and which may even include network points using different radio access technologies (RATs) such as high speed packet access (HSPA) NBs and WiFi access points. A NodeB may be associated with a point or multiple points, and a cell may include a point or multiple points, with each point having a single or multiple antennas. A point may also correspond to multiple cells operating in multiple component carriers. The eNBs are interconnected with each other by means of an X2 interface. The eNBs are also connected by means of an S1 interface to a Mobility Management Entity (MME) and to a Serving Gateway (S-GW). Additionally, a cell or NB may be serving a number of users (also commonly referred to as User Equipment (UE), mobile stations, terminals, devices, and so forth) over a period of time.
Generally speaking, in orthogonal frequency division multiplexing (OFDM) systems, the frequency bandwidth of the system is divided into multiple subcarriers in the frequency domain. In the time domain, one subframe is divided into multiple OFDM symbols. The OFDM symbol may have a cyclic prefix to avoid the inter-symbol interference caused by multi-path delays. One resource element (RE) is defined by the time/frequency resource within one subcarrier and one OFDM symbol. In a downlink transmission, reference signals (RSs) and other signals such as a data channel (physical downlink shared channel (PDSCH)), a control channel (physical downlink control channel (PDCCH)), and an enhanced PDCCH (EPDCCH) are orthogonal and multiplexed in different resource elements in the time/frequency domain. In an uplink transmission, physical uplink shared channel (PUSCH) and physical uplink control channel (PUCCH) are orthogonal and multiplexed in different time/frequency resources. A set of REs are grouped together to form a resource block (RB). For example, 12 subcarriers in a slot make up a RB.
Generally, to enable any data channels in either uplink (UL) or downlink (DL) transmissions such as PDSCH or PUSCH of an LTE-A system, reference signals are transmitted. There are reference signals for a UE to perform channel/signal estimation/measurements for demodulation of PDCCH and other common channels as well as for some measurements and feedback, which is the Common/Cell-specific Reference Signal (CRS) inherited from the Release 8/9 specification of Evolved Universal Terrestrial Radio Access (E-UTRA). A Dedicated/Demodulation Reference Signal (DMRS) can be transmitted together with the PDSCH channel in Release 10 of E-UTRA. DMRS is used for channel estimation during PDSCH demodulation. In Release 10, the Channel State Information Reference Signal (CSI-RS) is introduced in addition to CRS and DMRS. CSI-RS is used for Release 10 UEs to measure the channel status, especially for multiple antennas cases. PMI/CQI/RI and other feedback information may be based on the measurement of CSI-RS for Release 10 and beyond UEs. PMI is the precoding matrix indicator, CQI is the channel quantity indicator, and RI is the rank indicator of the precoding matrix. CSI-RS in Release 10 can support up to 8 transmission antennas while CRS can only support a maximum of 4 transmission antennas in Release 8/9. The number of CSI-RS antenna ports can be 1, 2, 4, or 8. In addition, to support the same number of antenna ports, CSI-RS has much lower overhead due to its low density in time and frequency.
A heterogeneous network (HetNet) comprises high power macro points and various lower power points that generally may share the same communication resources. The lower power points may include, but are not limited to, picos, micros, remote radio heads (RRHs), femtos (or home eNBs (HeNBs)), access points (APs), distributed antennas (DAs), relays, and near field communication points.
A network also may comprise several component carriers operating in different frequency bands. High frequency bands generally have a high pathloss over distance so they are more suitable to serve a relatively smaller area, such as being used for high throughput purposes for nearby UEs. Low frequency bands generally have low pathloss over distance so they are more suitable to serve a relatively large area, such as being used for providing coverage.